This invention relates to a catalytic gas detector, and to a method of manufacturing such a detector.
Catalytic gas detectors are used in the continuous monitoring of atmospheres to detect the presence of a low molecular weight flammable gases, such as methane. The principle of operation relies on the fact that small concentrations of hydrocarbon gases are cationically oxidized in air at elevated temperatures over a suitable catalyst such as palladium rhodium, platinum or iridium. The resultant exotherm reaction produces a temperature rise which is sensed by a sensor. There is a direct relationship between the degree of temperature rise and the proportion of combustible gas in the atmosphere being measured.
Sensors of the type described, which are commonly known as pellistors, comprise a coil of a noble metal, such as platinum, covered with a bead of porous catalytic material. The change in temperature referred to above leads to a change in resistance of the coil, which is detected in a Wheatstone bridge-type configuration of which the coil forms a part.
In the case of portable sensing instruments which are reliant on battery power, it is important that the power consumption of the pellistor be as low as possible.
There are two ways of achieving this end:
(i) By increasing the electrical resistance of the wire; PA1 (ii) By decreasing the amount of heat that is lost from the pelestor to its surroundings; PA1 a) a heating element; PA1 b) a temperature sensing element in the form of a layer-deposited electrically conductive track terminating in at least two electrically conductive bridging leads; PA1 c) a catalytic bead associated with the temperature sensing element, PA1 a) providing a substrate; PA1 b) depositing a heating and temperature sensing element in the form of at least one conductive track onto the substrate; PA1 c) etching away that portion of the substrate directly beneath the conductive track so as to define a cavity over which the conductive track is suspended, with bridging conductors extending between the conductive track and terminals on the substrate; and PA1 d) depositing a catalyst with a refractory carrier onto the conductive track so as to form a sensing structure supported by the bridging conductors. PA1 a) depositing a conductive track onto a substrate; PA1 b) preparing a catalyst comprising a noble metal on a refractory carrier by treating the catalyst with a catalyst precursor so as to form a slurry or paste; PA1 c) applying the slurry or paste to the conductive track by screen printing or stenciling; and PA1 d) calcining the slurry or paste in air.
In order to satisfy the first requirement, published UK patent application GB2238617A describes a thin wire in the form of a conductive track deposited on a non-conductive and thermally isolating layer, such as silicon nitride or silicon oxde, which is deposited upon the silicon substrate. Photolithographic processes are commonly used to define the conductive track or heater element. The bead is usually formed by the deposition of a material such as porous alumina on top of the heater element.
In order to satisfy the second requirement, the substrate beneath the isolating layer is generally etched away so that a suspended membrane is formed upon which the heater element is located, thereby increasing the thermal isolation of the heater element, which results in a significant decrease in power consumption. This is important for portable sensing instruments which are reliant on battery power.